Miniature broad band low pass filters with multiturn tape wound inductors

ABSTRACT

A miniature broadband high frequency low pass filter having at least one multiturn tape wound core inductor and at least one ceramic capacitor whereby the resultant filter exhibits improved insertion loss characteristics even in filter applications having certain amounts of DC or low frequency current passing therethrough.

United States Patent Peter A. Dene:

9101 Creitvood Ave. N.E., Albuquerque, N. Mex. 871 12 July 14, 1969Sept. 7, 1971 Continuation-impart of application Ser. No. 393,946, Sept.2, 1964, now Patent No. 3,456,215 and a continuation-impart of Ser. No.831,142,,Iune 6,1969

lnventor Appl. No. Filed Patented MINIATURE BROAD BAND LOW PASS FILTERSWITH MULTITURN TAPE WOUND INDUCTORS 10 Claims, 7 Drawing Pip.

US. Cl 333/79, 317/242, 317/260, 336/96 Int. Cl. H0311 7/02 Field ofSearch 333/30, 76,

[56] References Cited UNITED STATES PATENTS 3,260,972 7/1966 Pusch333/84 3,141,145 7/1964 Barrett 333/79 3,456,215 7/1969 Denes 333/793,219,951 11/1965 Clark 333/79 2,440,652 4/1948 Beverly 333/31 C2,973,490 2/1961 Schlicke 333/79 2,918,633 12/1959 Schenker.... 333/703,329,911 7/1967 Schlicke 333/79 Primary Examiner-Herman Karl SaalbachAssistant Examiner-C. Baraff Attorney-Spensley, Horn and LubitzABSTRACT: A miniature broadband high frequency low pass filter having atleast one multitum tape wound core inductor and at least one ceramiccapacitor whereby the resultant filter exhibits improved insertion losscharacteristics even in filter applications having certain amounts of DCor low frequency current passing therethrough.

MINIATURE BROAD BAND LOW PASS FILTERS WITH MULTITURN TAPE WOUNDINDUCTORS This application is a continuation-in-part of my copendingapplication Ser. No. 393,946 filed Sept. 2, 1964 which issued into U.S.Pat. No. 3,456,215 and which teaches the use of tape wound inductorcores having an insulating binder; and of my copending application Ser.No. 831,142, entitled High Frequency Low Pass Filter with EmbeddedElectrode Structure" which teaches the use of a multitum tape wound coreinductor inside of a double-capacitor of a pi filter, filed June 6, 1969BACKGROUND OF THE INVENTION 1. Field of the Invention The presentapplication relates generally to the field of miniature filters forbroadband high frequency low pass applications.

II. Description of the Prior Art In the past high frequency low passfilters have been constructed utilizing inductors having laminated orpulverized ferromagnetic metal cores (dust cores) or ferrite cores. Manyof the filter structures also used ceramic capacitors in combinationwith the core structures previously mentioned. However, the corematerials and core structures had certain per formance deficiencies anddisadvantages which greatly limited their use in high frequency low passfilter applications.

For example, laminated core structures made of obtainable laminathicknesses exhibit poor frequency response characteristics and theirpermeability generally starts to diminish quite drastically atfrequencies of about 0.1 MHz. which is in frequency range where highinductances are normally required for high frequency low pass filterapplications.

If it were possible to make laminated cores of metal laminae of the samethickness as the thickness of the tape, the frequency dependence of thepermeability would be substantially the same. However, no practicalmethod for making laminated ring stack cores is presently known. Forexample, to obtain a 0.3-inch long core would require approximately2,400 rings or each 0.000125 inch in thickness. The presently knowntechniques would make fabrication of such a core awkward and completelyuneconomical. As another example, dust cores either exhibit apermeability of about 500 in the low kilohertz ranges, but thepermeability decreases sharply at about 50 kHz. or if operation isrequired in megahertz frequency range the permeability of the core islimited to below about 100.

While ferrite cores have reasonably high permeabilities (in the range ofabout 3,000 and above) up to the few hundred kilohertz ranges, theirsaturation induction is quite low (e.g. 3,000 to 5,000 Gauss).Therefore, the permeability of a miniature multitum ferrite may drop toabout at a field strength of about 3 Oersted which may correspond to acurrent as low as 0.01 amperes, for example. Generally, broadband lowpass filters are used in circuit applications where certain amounts ofDC or low frequency current must pass. The effectiveness of the priorart filters is usually completely diminished by current bias when inorder to obtain miniaturized filters the prior art type of inductorcores (e.g. ferrites) of higher permeabilities are employed.

SUMMARY OF THE INVENTION found that the permeability and especially thevalue /1 +Q which is generally considered to be the most importantinductor characteristic regarding filters is very stable up to themegahertz ranges if thin tapes, (e.g. 0.000125 inch thick) are wound andused. In the above recited formula, t stands for the penneability and Qis the quality factor. The value defined previously drops very littleeven at higher frequencies, up to about 50 MHz., after which it beginsto decrease more rapidly. Such perfonnance was unobtainable withpreviously used laminated metal or ferrite cores. It should be notedthat if a high permeability is needed only at lower frequencies, thethickness of the tape can be as great as 0.001 inch or more. On theother hand, if the high permeability must be maintained at higherfrequencies, the thinnest tape obtainable today which is about 0.00004inch thick may be used. Various configurations of invented filters (e.g.L-type, pi-type, T-type, double L-type, double Pi-type, double T-typeetc.) can be made utilizing separate ceramic capacitors and themultiwound tape wound core inductors taught by the inventive conceptdisclosed herein.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional end view of atape wound core having a plurality of turns wound therethrough;

FIG. 2 is a cross-sectional, front elevational view which illustrates anL-type filter utilizing a multitum tape wound core inductor;

FIG. 2a is a schematic diagram of the filter shown in FIG. 2;

FIG. 3 is an enlarged cross-sectional view of one form of a multilayerceramic capacitor which may be utilized in the invention and is shownalso in FIG. 2;

FIG. 4 is a cross-sectional front elevation view which illustratesanother embodiment of the invented filter in which multitum tape woundcore inductors are used in a pi-type filter;

FIG. 4a is a schematic diagram of the filter shown in FIG. 4; and

FIG. 5 is a cross-sectional front elevation view illustrating anL-section feedthrough filter having a multiturn tape wound core and asingle ceramic capacitor envelope.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It has been found that for DCor low frequency current (e.g. 60 to 400 c.p.s.) carrying miniature lowpass filters, multitum tape wound cores yield superior performancecompared with any of the filters using inductors made according topresently known techniques.

Tape wound core inductors for use in filters were disclosed in US. Pat.Ser. No. 3,456,215. In this patent single turn tape wound cores wereemployed in pi filters, inside of a ceramic double-capacitor. Anothercopending application of mine (Ser. No. 831,142, filed June 6, 1969)entitled High Frequency Low Pass Filter with Embedded ElectrodeStructure," discloses the use of Multiturn tape wound core inductorsinside of the double-capacitor of a pi filter.

The present invention utilizes filters with individual ceramiccapacitors and hollow multitum tape wound core inductors which makepossible the building of broad band high frequency low pass filters ofvarious types, for example, L-type, pitype, T-type, double L-type,double pi-type, double T-type, etc.

In theory, the tape material for the core may be any ferromagnetic metalor alloy. However, in practice, those magnetic materials are chosenwhich have a permeability suitable to the required characteristics ofthe filter, and exhibit as high a saturation induction as possible. Thethickness of the material will generally be detennined by the desiredpermeability versus frequency performance. While a laminated core using0.004 inch thick laminates (about the lowest practical limit) may startto drop its permeability rapidly from 20 kHz. upwards, a tape wound coree.g. witha tape thickness 0.000125 inch would roughly maintain theoriginal permeability up to about 2 MHz. Iron, cobalt, and iron-cobaltalloys are especially favored materials because their saturationinduction is about 21,000 Gauss or higher. However, for specialpurposes, other known alloys, like e.g., iron-nickel,iron-cobalt-nickel, iron-nickel-molybdenum, etc. would also be employed.If desired nonferromagnetic metal may be alloyed with ferromagneticmaterials of the general type previously mentioned. The addition of thenonferromagnetic material may allow improvements of certain inductorproperties. For example, the permeability may be increased and/or the Qfactor increased or diminished depending on alloys used and thecomposition required to obtain the desired results. FIG. 1 shows ahollow tap wound core formed from a ferromagnetic tape 11 (e.g. thematerials description above) with an insulation layer 12 (e.g. asuitable binder of epoxy resin or a very thin, e.g. 0.000002 inch thick,oxide layer) between the layers of tape 11. A single length of wire W iswound through the core and around the outer surface thereof to provide amultiturn structure as shown in FIG. 1. As will be discussed below thetype of tape material and the number of wire turns can be selected tosuit the various application requirements for inductors in broadband lowpass filters.

Ceramic capacitors which may be utilized in these filters are known perse in the art and are not a part of this invention. While in principlethe capacitors can be a single ceramic tubular or discoidal capacitorgenerally a plurality of such tubes or discs are connected in parallelto obtain the needed capacitance values. For higher voltages, often tubeclusters or disc stacks may be built up from single self-sustainingcapacitors. In practice, generally multilayer ceramic capacitors areused which may be in the form of ceramic rolled capacitors, multidisccapacitors or multitubular capacitors as described in my copendingapplication entitled "High Frequency Low Pass Filter with EmbeddedElectrode Structure," filed June 6, 1969 (our File PD-l072-387 CIP (A)In order to illustrate the superior performance characteristics obtainedby using the inventive concept the following structural examples arepresented.

EXAMPLE I With reference to FIG. 2, an L-type filter 13 is shown havinga standard enclosure 14, a tape wound core inductor 10 (as describedpreviously) and a ceramic multilayer capacitor 15. Typically, theenclosure 14 and parts of the enclosure may be made from metal such assteel or copper ad has openings I6 and 17 therein. The tape wound coreinductor 10 is positioned so that the ends W and W of the wire W whichis wound around and through the inductor 10 pass trough openings 16 and17 insulated from parts of the enclosure and form external terminals forcircuit connections. The wire end W is also connected at point 18 to thecapacitor at the electrode connecting washer 19. The capacitor 15 istypically formed by a number of parallel embedded electrodes and 21(e.g. palladium or other noble metals) which are insulated from eachother by ceramic layers C. One set of parallel electrodes 20 are theplate forming electrodes of the capacitor and are connected together byouter conductive layers 20a and 20b to the electrode connecting washer19 for connection to the wire end W of the inductor. (See FIG. 3 Theouter set of parallel electrodes 21 are the ground forming electrodeswhich are connected together by outer conductive layers 21a and 21b tothe electrode connecting washer 22 for connection to the enclosure 14.The enclosure 14 is generally at some base potential such as groundpotential. The washer 22 is generally joined to or in contact with thecasing at point 23. The electrodes 20 and 21 are physically andelectrically insulated from each other as are their respectiveconnecting conductive layers 20a, 20b, and 21a and 21b, respectively, asshown in FIG. 3. The wire end W passes through an opening 22a in theconnecting washer 22 without making any physical or electrical contactwith washer 22.

Normally, the filter 13 is hermetically sealed at points 224 and 25using known techniques and the internal spaces between the components ofthe filter in the interior of the enclosure 14 are encapsulated with asuitable potting material 26 such as an epoxy material or siliconerubber. The hermetic sealing and potting of the filter give it bettermechanical and vibration resistant characteristics. A schematic diagramof the filter structure of FIG. 2 is shown in FIG. 2a, using thestructure shown in FIG. 2.

By way of example, a miniature 50 VDC L-type filter having the structureshown in FIG. 2 can be built to obtain the improved operatingcharacteristics previously described. The capacitor 15 would have acapacitance of l mfd. and the tape wound core 10 may be made of Armcoiron, a product of Armco Steel Co. The tape 11 of which the core 10 isformed should be about 0.00025 inch thick. The initial permeability ofthis core material is approximately 1,000 up to 1 MHz. and thesaturation induction is about 21,500 Gauss. The core I0 is wound with 25turns of wire W and its inductance should be about 500 microhenries. Theattenuation of the filter 13 should be 25 db. at 30 KHz., 45 db. at I00kHz. and db. at 1 MHz. Utilizing the multiturn tape wound core describedhereinabove, in the filter structure shown in FIG. 2, the attenuationfigures previously recited should not drop appreciably even with DC orlow frequency (60 to 400 c.p.s.) tape 11. loads of up to W amps. A 50VDC L-type filter 13 as described could have a cylindrical enclosure 14which would measure only 0.3 inch in diameter and 0.5inch in length.

EXAMPLE II With reference to FIG. 4, a pi-type filter 30 is shown havinga standard enclosure 31, a tape wound core inductor 10' (of similarstructure to that previously described) and two multilayer ceramiccapacitors 15' and 15 The enclosure 31 can be of a similar type to thatdescribed for enclosure 14. The tape wound core 10' is positioned sothat the ends W, and W of the wire W which is wound around and throughthe inductor 10 pass through the openings 32 and 33 and form externalterminals for circuit connections. The wire ends W, and W are alsoconnected at points 34 and 35 to the capacitors l5 and 15",respectively, at the electrode connecting washers 36 and 37. Thecapacitors l5 and 15" may be formed by a number of parallel electrodes20 and 21' in capacitor 15 and 20" and 21" in capacitor 15". The variousset of capacitor electrodes are insulated from each other by layers ofceramic material C and C", respectively. One set of parallel electrodes20 and 20" are plate forming electrodes of the capacitors l5 and 15",respectively, and are connected by outer conductive layers 20a and 20a"to electrode connecting washers 34 and 35 for connection to the wireends W, and W The other set of parallel electrodes 21 and 21" are groundforming electrodes of the capacitors l5 and 15", respectively, which areconnected together by outer conductive layers 21a and 21a" to electrodeconnecting washer 38 and 39 for connection to the enclosure 31. Theenclosure 31 is generally at some base potential such as groundpotential. The washers 38 and 39 are typically joined to or in contactwith the casing at points 40 and 41. The capacitors shown in FIG. 4 areslightly different in configuration from those shown in FIGS. 2 and 3(e.g. the electrode orientation) but operate in substantially the samemanner. Either configuration may be used to obtain excellent results inthe various filters described.

The wire ends W, and W pass through the openings 38a and 39a in theconnecting washers 38 and 39, respectively, without making any physicalor electrical contact with the washers 38 and 39.

Normally, the filter 30 is hermetically sealed at points 42 and 43 usingknown techniques and the internal spaces between the components of thefilter in the interior of the enclosure 31 are encapsulated with asuitable potting material 44 such as an epoxy material or siliconerubber. A schematic diagram of the filter structure of FIG. 4 is shownin FIG. 4a

Using the structure shown in FIG. 4, a v. DC miniature pitype filter canbe built as described below. The multitubular capacitors 15' and 15"each would have capacitance of l mfd. The tape wound core 10' may bemade of a 50 percent iron and 50 percent cobalt containing alloy. Thethickness of the tape used should decrease about 0.001 inch. The initialpermeability of this core 10' is approximately 800 up to 4 MI-Iz., thesaturation induction is about 24,500 Gauss. The core is then wound withturns of wire W. The inductance should be about 160 microhenries. Theinsertion loss of the filter 30, measured according to MIL-STD-220 A.,should be 28 db. at I00 kHz. and 100 db. about 500 kHz. The

insertion loss will not decrease with a through flowing DC or lowfrequency current (60 to 400 c.p.s.) of 1.5 amps. The miniature pi-typefilter 30 described may be mounted in a cylindrical enclosure 31 assmall as 0.32 inches in diameter and 0.65 inches in length. As acomparison, if a filter of the same performance and size were built witha ferrite core, a DC or low frequency current higher than 0.2 amps wouldstart to appreciably decrease its insertion loss.

The structure shown in FIGS. 2 and 4 are illustrative of two types offilters which may be formed using the same basic type of multiturn tapewound inductor in combination with at least one ceramic capacitor. Morethan one inductor may be used for example in a T-type filter. Also, aplurality of inductors and capacitors may be used for example in adouble-L-type filter. The various structures for these and other lowpass filters in accordance with the teachings described and taught forthe filters in FIGS. 2 and 4 would be apparent to those skilled in theart. The following configurations of filters using the inventive conceptmay also be built to give superior results when compared with any of theknow filters of similar size using the prior art inductor corespreviously described.

EXAMPLE III A miniature T-type filter may be made with a 50 v. DCmultilayer capacitor of l mfd. and two identical multitum tape woundcore inductors. Each inductor could have a tape wound core fabricated ofa 65 percent iron and 35percent cobalt from a tape 0.00025 inch thick.The initial permeability should be about 300 and the saturationinduction about 23,500 Gauss. The winding of the core would have 30turns and an inductance of about 240 microhenries. The insertion loss ofthe filter should be 24 db. at 30 kHz. 55 db. at 100 kHz.

and more than 90 db. above 500 kHz. when measured accord ing toMIL-STD220A The insertion loss will not decrease when a 2amp. DC or lowfrequency current (60 to 400 c.p.s.) flows through the filter. The sizeof the enclosure for this filter would be as small as 0.3 inch indiameter and 0.7 inch in length.

EXAMPLE IV A miniature 50 V. DC double-L-type filter may be constructedwith two ceramic multilayer capacitors, each I mfd., and two identicalmultiturn tape wound core inductors. The tape may be 0.00025 inch thickand its material may be an alloy composed of 45 percent nickel and 55percent iron. The initial permeability of this alloy is approximately3,500, but its saturation induction is only 16,000, which is still muchmuch higher than that of ferrites. The core is wound with turns and theinduction should be approximately 1,200 microhenries. The attenuation ofthis filter should be 65 db. at kHz. and more than 100 db. above 100kHz. The size of the enclosure for such a filter can be as small as 0.3inch in diameter and 1 inch in length. The maximum DC or low frequencycurrent (60 to 400 c.p.s.) which would not lower the attenuation is only0.1 amp. in this embodiment. As a comparison, however, if a filter ofthe same performance and same size would have been built with a ferritecore, the maximum DC or low frequency current not lowering theattenuation would be only approximately 0.025 amp. Further, if a filterof the same attenuation performance had been built with a known ferriteinductor core in order to carry currents up to 0.1 amp. withoutappreciably decreasing the attenuation, the volume of the necessaryfilter enclosure would be approximately five times larger than theenclosure described hereinabove.

EXAMPLE V An L-section feedthrough configuration filter as shown in FIG.5 has been found to give similar performance to filters which were thesmallest available prior to the filter embodiments taught by the presentinvention. However, the smallest prior art filters required volumesapproximately twenty times greater than the volume of the filter shownin FIG. 5 and described hereinbelow.

The envelope of the filter 50 is a single multitubular capacitor 51(ceramic) which can be soldered to a planar ground member 52 as afeedthrough unit. The capacitor 51 is cylindrical and is made of aplurality of electrodes embedded in a ceramic insulating material C'.The electrodes 53 may be ground fonning electrodes and areinterconnected by a conductive (e.g., metal) layer 54 which is placed incontact with the member 52. The member 52 may be made from any suitableconductive material, typically a metal such as copper or steel. Theplate electrodes alternate with the ground electrodes 53 and 55 and areinsulated from the latter electrodes by layers of the ceramic materialC' located between the various plate and ground electrodes. The plateelectrodes 55 are connected to each other by a conductive (e.g., metal)layer 56.

A hollow ferromagnetic tape wound core inductor 10" of the structureshown in FIG. I and described above, is wound with turns of wire W". Thewire W is wound around and through the inductor 10" and one end W," ofthe wire W passes through an opening 52a in the member 52 to form anexternal connection point for the filter. The end of the wire W," is inelectrical and physical contact with the member 52 at point 52b and alsoin contact with the ground electrodes 54. The end W of the wire W" isnot in electrical contact with the capacitor 51 and forms a secondexternal connection point. This inductor-capacitor structure forms anL-Section feedthrough filter since the inductor is in contact with onlyone side of the single capacitor as described above. A suitable pottingmaterial 57 may be used to encapsulate the components to keep them inposition and to give good mechanical properties to the filter.

Using the structure shown in FIG. 5 a miniature L-section feedthroughfilter may be made for example, as follows:

The inductor 10" is a tape wound core made of a composition of 17percent iron, 79 percent nickel and 4 percent molybdenum. Thickness ofthe tape is 0.000125". The number of the turns of wire W" is 4. Thepermeability of this material is about 15,000 at 150 kHz., which isstill much higher than any other type of magnetic material, e.g.ferrite. However, the saturation induction is only 8,000 Gauss, hencethe DC or low frequency current which will not influence theattenuation, is not as high as in the examples previously describedabove.

The dimensions of the filter 50 were only 0.120 inch in diameter and 0.4inch in length. The ceramic capacitor 51 had a capacitance of 0.2 p/F.and a working voltage of 50 v. DC. The inductance was aboutmicrohenries. The attenuation at 0.15 MHz. was 25 db. and at 1 MHz. 50db. Such filters yield a very dense multipaekage for filtering a numberof through going connections between miniature integrated subsystems,and yet give very good attenuation performance at the lower end of thefrequency spectrum too.

Other miniature low pass filter configurations may be made within thescope and spirit of the invention by those skilled in the art. Thespecific configurations described in this specification are merelyexemplary of the inventive concept and technique and are not to beconsidered as limitations of the invention.

I claim:

I. A broadband low pass filter having at least one multilayer ceramiccapacitor therein comprising:

a. at least one inductor having a hollow ferromagnetic tape wound corewith a insulating layer between the tape windings of said core, the tapewindings of said core being disposed upon one another with negligibleaxial displacement; and

b. a plurality of wire windings formed from a single wire of a givenlength inserted through said hollow inductor and around said inductor toform a multitum inductor which is operatively connected in said filter.

2. The low pass filter described in claim 1 in which said tape woundcore is formed from a ferromagnetic material selected from the groupconsisting of iron, cobalt and alloys of ironcobalt and alloys of thesaid group with nonferromagnetic metals.

3. The low pass filter described in claim 1 in which said tape woundcore is formed from a ferromagnetic material selected from the groupconsisting of alloys of iron-nickel, cobaltnickel andiron-cobalt-nickel, and alloys of the said group with nonferromagneticmetals.

4. The low pass filter described in claim 1 in which said tape woundcore is formed from a tape having a thickness in the range ofapproximately 0.00004 inch to 0.001 inch.

5. The low pass filter described in claim 2 in which said ferromagneticmaterial has a saturation induction in excess of about 16,000 Gauss.

6. A broad band low pass filter comprising:

a. a metal enclosure;

b. at least one ceramic multilayer capacitor located in said enclosure;

c. at least one multitum ferromagnetic tape wound core operativelyconnected to said capacitor to form a given filter circuit, the tapewindings of said core being disposed upon one another with negligibleaxial displacement; and

d. said multitum core having a single wire wound through and around saidcore with the wire ends extending through said enclosure to formexternal connection points for said filter.

7. The low pass filter described in claim 6 in which said tape woundcore is formed from a ferromagnetic material selected from the groupconsisting of iron, cobalt and alloys of ironcobalt and alloys of thesaid group with nonferromagnetic metals.

8. The low pass filter described in claim 6 in which said tape woundcore is formed from a ferromagnetic material selected from the groupconsisting of alloys of iron-nickel, cobaltnickel and iron-cobalt-nickeland alloys of the said group with nonferromagnetic metals.

9. The low pass filter described in claim 6 in which said tape woundcore is formed from a tape having a thickness in the range ofapproximately 0.00004 inch to 0.001 inch.

10 The low pass filter described in claim 7 in which said ferromagneticmaterial has a saturation induction in excess of about 16,000 Gauss.

1. A broadband low pass filter having at least one multilayer ceramiccapacitor therein comprising: a. at least one inductor having a hollowferromagnetic tape wound core with a insulating layer between the tapewindings of said core, the tape windings of said core being disposedupon one another with negligible axial displacement; and b. a pluralityof wire windings formed from a single wire of a given length insertedthrough said hollow inductor and around said inductor to form amultiturn inductor which is operatively connected in said filter.
 2. Thelow pass filter described in claim 1 in which said tape wound core isformed from a ferromagnetic material selected from the group consistingof iron, cobalt and alloys of iron-cobalt and alloys of the said groupwith nonferromagnetic metals.
 3. The low pass filter described in claim1 in which said tape wound core is formed from a ferromagnetic materialselected from the group consisting of alloys of iron-nickel,cobalt-nickel and iron-cobalt-nickel, and alloys of the said group withnonferromagnetic metals.
 4. The low pass filter described in claim 1 inwhich said tape wound core is formed from a tape having a thickness inthe range of approximately 0.00004 inch to 0.001 inch.
 5. The low passfilter described in claim 2 in which said ferromagnetic material has asaturation induction in excess of about 16,000 Gauss.
 6. A broad bandlow pass filter comprising: a. a metal encLosure; b. at least oneceramic multilayer capacitor located in said enclosure; c. at least onemultiturn ferromagnetic tape wound core operatively connected to saidcapacitor to form a given filter circuit, the tape windings of said corebeing disposed upon one another with negligible axial displacement; andd. said multiturn core having a single wire wound through and aroundsaid core with the wire ends extending through said enclosure to formexternal connection points for said filter.
 7. The low pass filterdescribed in claim 6 in which said tape wound core is formed from aferromagnetic material selected from the group consisting of iron,cobalt and alloys of iron-cobalt and alloys of the said group withnonferromagnetic metals.
 8. The low pass filter described in claim 6 inwhich said tape wound core is formed from a ferromagnetic materialselected from the group consisting of alloys of iron-nickel,cobalt-nickel and iron-cobalt-nickel and alloys of the said group withnonferromagnetic metals.
 9. The low pass filter described in claim 6 inwhich said tape wound core is formed from a tape having a thickness inthe range of approximately 0.00004 inch to 0.001 inch. 10 The low passfilter described in claim 7 in which said ferromagnetic material has asaturation induction in excess of about 16,000 Gauss.